Treatment of toxic copper-containing solutions



United States l atent (3 i 3,21s,2s4 TREATNIENT F TOXIC COPPER-0NTAINING SOLUTEDNS Leslie E. Laney, Ellwood City, Pa, assignor to Laney Laboratories, lino, Zelienople, Pin, a corporation of Pennsylvania No Drawing. Continuation of application Ser. No. 18,240, Mar. 29, 1960. This application Nov. 20, 1962, Ser. No. 239,091

8 (Iiaims. (Cl. 210-48) This application is a continuation of my application No. 18,240 filed March 29, 1960, entitled Treatment of Toxic Copper-Containing Solutions and abandoned in favor of the present application.

This invention relates to the treatment of waste or spent copper-containing solutions, such as spent ammonium persulfate copper etching solutions, to make them suit-able for discarding, for example, into a sewage system in form of non-toxic Waste.

An ammonium persulfate solution is used as an effective and fast etchant for copper and brass at temperatures from room temperature to 150 -F. The active ammonium persulfate is consumed by the etching process and when the solution becomes saturated with copper, the etchant solution has to be discarded. Such an etching solution is highly toxic even at a very low concentration, due essentially to its copper salt content and is thus a proper subject for waste treatment.

Neutralization of the free acid in the spent solution constitutes an insuflicient treatment, since the copper salts are soluble in the ammonia compounds remaining therein. The presently accepted and employed method of treatment involves a two-stage process. In the first stage, the copper is progressively removed from the solution by electrodepositing it on cathode sheets employed with insoluble anodes. When the copper is as completely as possible removed by electrolysis, the second stage of the treatment is performed and involves neutralization of the free acids in the remaining solution with alkali metal hydroxides of carbonates.

The above has been found to be a cumbersome and expensive type of waste treatment, and particularly from the standpoint of its first stage. In its first stage, it requires electrolysis facilities, such as an electroplating tank with heating and cooling coils, large cathode and anode areas, and the consumption of considerable direct current electrical power.

In order to remove the copper by the first stage to a low remaining concentration, it is necessary to provide copious solution agitation on the cathode surface and means therefor, as well as adjustments in the applied current density and means therefor. Much lower current densities are required for the remaining lower concentration of copper after the majority has been plated out. Further, a test must be conducted, even after the first stage of treatment is considered complete, to determine if the solution then has the desired low copper concentration and, in any event, a minimum concentration of copper tends to remain in the solution, even with the most carefully conducted and controlled electrolysis operation.

I have also found that in employing the electrolysis of a low copper concentration in a highly acidic electrolyte such as involved, that the deposit of copper on the cathode tends to become rough with the formation of burrs and so-called trees. Some of these trees, particularly as they build-up, tend to drop off and re-dissolve in the acid electrolyte, thus defeating the desired purpose of completely removing the copper from the solution. In addition, coldrolled copper sheet has to be used for the starting electrode and thus, becomes part of the total cathode deposit and has to be resold at electrolytic copper scrap prices.

3,218,254 Patented Nov. 16, 1965 "ice Unsuccessful attempts have been made to improve the above-mentioned two-stage process, since toxic waste treatment is becoming more and more a requirement in various communities. Those skilled in the art have heretofore been unable to meet the requirements for an inexpensive, practical and more efiicient treatment process which will make the toxic solution innocuous or non-toxic for free discarding. However, I have been able to meet this problem by an entirely new approach to the factors involved and to do it in such a manner that even toxic low copper concentrations are eliminated.

It has thus been an object of my invention to determine and evaluate the factors involved in the treatment of ammonium persulfate toxic copper solutions and to meet such factors in an improved, simplified and effective procedure;

Another object of my invention has been to provide a method of treating such a toxic solution as to minimize the equipment involved, the tedious controls and tests heretofore required, and to do so by an entirely new approach to the problem;

A further object of my invention has been to devise a treatment for a copper-containing toxic waste solution of an improved, more effective and complete type;

These and other objects of my invention will appear to those skilled in the art from the description of my process and the appended claims.

The invention relates to a process for the treatment of a used etching solution containing toxic copper salts and ammonium persulfate to convert the solution from a toxic to a non-toxic condition for freely discarding as non-toxic Waste. It involves adding an excess of caustic soda to the toxic solution and heating the solution, to convert the ammonium and copper content into ammonia gas and copper oxide, and to drive off the ammonia gas and precipitate out or remove the copper oxide from the solution to form a copper-free and ammonia-free supernatant liquid.

In carrying out my process, I entirely eliminate the need for plating-out the copper content of a spent ammonium pers-ulfate etchant solution by first adding an excess of a strong neutralizing alkali, such as caustic soda, preferably in solution form, to the spent etchant to provide a solution having a minimum pH of 12. The thus-treated solution is then conditioned or heated and preferably brought to a boiling temperature to drive-out its ammonia content. The caustic soda is employed to convert the ammonium sulfate compounds into ammonium hydroxide which is then driven-out of the solution by the heating operation in the form of ammonia gas. During the heating, caustic soda additions may be made to maintain the excess caustic soda needed, since some caustic soda is consumed for the neutralization and precipitation of the copper as the ammonia is driven out.

I have discovered that the vapor pressure of the ammonia gas is sufiiciently high to complete the reaction in about one hour at the boiling temperature of the solution, for example, in the neighborhood of C. Heating, however, may be effected at a lower temperature for a slightly longer time, depending on the temperature of the solution. A good indication that all the ammonium compounds are converted into ammonium hydroxide is the fact that free caustic soda is available. In this connection, the pH of a solution containing about .4 gram per liter of NaOH is 12, thus a pH of 12 indicates complete conversion. As the copper is precipitated, some further caustic soda may be needed, as indicated by a pH test, but my usual procedure is to add a suflicient excess at the beginning of the treatment. The quantity of this excess will depend on the usual copper concentration of the solution and will be determined by the user in an empirical manner. It is thus apparent that free caustic soda should be available throughout the treatment.

3 It is well known that NH is a gas that, when dissolved in water, forms ammonium hydroxide The vapor pressure of ammonia gas at 25 C. is sufficiently low that the aqueous tension is in balance with the vapor pressure of the gas up to the maximum solubility limit or" 480 grams per liter of ammonia. At this high concentration, there is a continuous loss of ammonia from the solution and therefore, the commercial available concentration is only 280 grams per liter. The lower the concentration, the greater the tendency of the aqueous tension to overcome the vapor pressure of the ammonia gas which is considerable, even at room temperature. As the solution temperature is increased, the vapor pressure increases and thus, the solubility of the ammonia in water becomes less and less. By way of example, the vapor pressures of ammonia at different temperatures are about atmospheres at C., about 31 atmospheres at 68 C., about 41 atmospheres at 80 C., and about 62 atmospheres at 100 C. The practical result is that at a boiling temperature, the ammonia gas may be driven out of solution in about one hour. At 80 C., the time required will increase by about 50 to 100%.

Although, as above indicated, my treatment can be carried out at lower heating temperatures of the solution that are above normal room temperature, boiling is preferable, since it produces a continuous agitating or stirring action that brings the dissolved ammonia closer to the solution air interface. As a result, the ammonia gas will have less water tension to overcome when escaping into the atmosphere. At lower temperatures when the stirring action of the boiling liquid is missing, mechanical stirring may be employed and, in fact, is needed.

A further consideration is that dehydration of the precipitated copper hydroxide to oxide, see Equations 2 et seq., is effected by heating the solution above room temperature, and while this reaction will proceed at a temperature lower than required for boiling, optimum results are obtained by boiling or by the use of a boiling temperature.

While the ammonium compound, namely, ammonia gas, is being removed from the solution due to vaporization-01f, the copper salts become insoluble and precipitate as cupric hydroxide. The maintenance of the heated or boiling solution causes the cupric hydroxide, itself, to decompose due to dehydration, so that cupric oxide is precipitated. Since cupric oxide is a relatively heavy compound having a specific gravity of 6.4 to 6.45, it settles rapidly and can be easily removed from the solution by settling, filtration or centrifuging. The remaining supernatant liquid consists essentially of sodium sulfate and sodium hydroxide which may be neutralized with a suitable acid, such as sulphuric acid, so that the solution can now be fully discarded as fully-treated non-toxic waste. 7

By way of example, the etching reaction of an ammonium persulfate solution, as heretofore employed in etching copper or copper in brass, is represented as follows:

It will be noted that from Equation 1 that the ammonium persulfate solution i substantially consumed by the reaction, in that its content is decreased from about 240 grams per liter to about 10 grams per liter. It will be further noted that, in accordance with heretofore employed conventional treatment, in which copper is removed by electrolysis, essentially only remains. However, in accordance with my treatment, procedure or method, ammonia gas is driven oif after neu- 4. tralization with caustic soda, such that Na SO +CuO (precipitated) still remains, as follows:

In the above equations, numbered 2 to 5a, those having the sufiix of a are secondary reactions due to the elevated temperature. I

It will thus appear that employing my method of treatment, the spent etchant solution need only be drawn 01f into a suitable heating tank or vessel, that the caustic soda solution can then be added to the solution of the tank and the solution heated to complete the process. The treated solution can be withdrawn from an intermediate level of the tank and the precipitated copper oxide may be removed after it has settled to the bottom of the tank. It is thus apparent that the process is simple as to its appartus requirements, is highly etficient, does not require meticulous controls and only involves a short period of time for effecting a complete removal of the toxic content.

What I claim is:

1. A process for the treatment of a used etching solution containing copper salts including a copper ammonium compound and a minor amount of ammonium persulfate to free the solution from its toxic ammonia and copper content for discarding the treated solution as Waste which comprises, adding an excess of a strong neutralizing alkali of the nature of caustic soda to the solution, maintaining an excess of the alkali in the solution throughout the treat-ment in an amount sufiicient to provide a minimum pH of about 12; heating the solution to an elevated temperature above room temperature and maintaining the excess of the alkali and the elevated temperature while converting its ammonium content into ammonia gas and driving-01f the ammonia gas, and while progressively with the driving-oil of the ammonia gas, freeing its copper con tent from its ammonium compound and converting its freed copper content into and precipitating it out of the solution as copper oxide; and continuing the defined treatment until the solution is essentially fully freed of its toxic ammonia and copper content.

2. A process for the treatment of a used etching solution containing copper salts including a copper ammonium compound and a minor amount of ammonium persulfate to free the solution of its toxic ammonia and copper content for discarding the treated solution as waste which comprises, adding an excess of a strong neutralizing alkali of the nature of caustic soda to the solution; heating the solution to an elevated temperature above room temperature and agitating it and maintaining the excess of the alkali to provide the solution with a minimum pH of about 12, while converting its ammonium content into ammonia gas and vaporizing-oft" the ammonia gas, and while progressively with the vaporizing-oil of the ammonia gas, freeing its copper content from its ammonium compound and precipitating its copper content out of the solution; and continuing the defined treatment until the solution is essentially fully freed of its toxic ammonia and copper content.

3. A process as defined in claim 2 wherein the supernatant liquid is thereafter neutralized as to it alkaline content before discharging it as waste.

4. A process as defined in claim 2 wherein further alkali is added during the treatment to maintain the minimum pH.

5. A process as defined in claim 2 where the solution is heated to and maintained at a boiling temperature of about 100 C. during the treatment.

6. A process as defined in claim 2 wherein the solution is stirred during the treatment.

7. A process as defined in claim 6 wherein the stirring is accomplished by heating the solution to and maintaining it at a boiling temperature during the treatment.

8. A process for the treatment of a used etching solution containing copper ammonium sulfate, copper sulfate, ammonium sulfate, sulphuric acid and a minor amount of ammonium persulfate in solution to essentially fully free the solution from its toxic ammonia and copper content for discarding the treated solution as waste which comprises, reacting the ammonium sulfate with a strong neutralizing alkali hydroxide and forming an alkaline sulfate and ammonium hydroxide, applying heat and agitation to the ammonium hydroxide to convert it to ammonia gas, and driving the ammonia gas out of the solution; reacting the copper ammonium sulfate with the alkali hydroxide to form an alkali sulfate, copper hydroxide and ammonia gas, and driving off the ammonia gas from the solution; applying heat to convert the copper hydroxide into copper oxide and water, and precipitating the copper oxide out of the solution; progressively with the driving oil of the ammonia gas, reacting the copper sulfate with the alkali hydroxide to form an alkali sulfate and copper hydroxide, and applying heat to break down the copper hydroxide into water and copper oxide, and precipitating the copper oxide out of the solution; reacting the ammonium persulfate with the alkali hydroxide to form an alkali persulfate and ammonium hydroxide, applying heat to the ammonium hydroxide to form water and ammonia gas, and driving off the ammonia gas from the solution; and continuing the defined treatment until the solution is essentially fully freed of its toxic ammonia and copper content.

References Cited by the Examiner UNITED STATES PATENTS MORRIS O. WOLK, Primary Examiner. 

1. A PROCESS FOR THE TREATMENT OF A USED ETCHING SOLUTION CONTAINING COPPER SALTS INCLUDING A COPPER AMMONIUM COMPOUND AND A MINOR AMOUNT OF AMMONIUM PERSULFATE TO FREE THE SOLUTION FROM ITS TOXIC AMMONIA AND COPPER CONTENT FOR DISCARDING THE TREATED SOLUTION AS WASTE WHICH COMPRISES, ADDING AN EXCESS OF A STRONG NEUTRALIZING ALKALI OF THE NATURE OF CAUSTIC SODA TO THE SOLUTION, MAINTAINING AN EXCESS OF THE ALKALI IN THE SOLUTION THROUGHOUT THE TREATMENT IN AN AMOUNT SUFFICIENT TO PROVIDE MINIMUM PH OF ABOUT 12; HEATING THE SOLUTION TO AN ELEVATED TEMPERATURE ABOVE ROOM TEMPERATURE AND MAINTAINING THE EXCESS OF THE ALKALI AND THE ELEVATED TEMPERATURE WHILE CONVERTING ITS ANNOMIUM CONTENT INTO AMMONIA GAS AND DRIVING-OFF THE AMMONIA GAS, AND WHILE PROGRESSIVELY WITH THE DRIVING-OFF TO THE AMMONIA GAS, FREEING ITS COPPER CONTENT FROM ITS AMMONIUM COMPOUND AND CONVERTING ITS FREED COPPER CONTENT INTO AND PRECIPITATING IT OUT OF THE SOLUTION AS COPPER OXIDE; AND CONTINUING THE DEFINED TREATMENT UNTIL THE SOLUTION IS ESSENTIALLY FULLY FREED OF ITS TOXIC AMMONIA AND COPPER CONTENT. 